The detackification of sticky materials in aqueous systems is a common problem in many industrial operations. Particularly in the automobile industry, the products are coated or painted in enclosed areas referred to as paint spray booths. A significant portion of the paint is oversprayed, that is, not transferred to the object being coated. Such waste paint is generally referred to as oversprayed paint, and is usually collected in water for subsequent waste treatment.
Paint is a tacky material and its tends to coagulate and adhere to the spray booth surfaces and must continuously be removed from the booth to avoid blockage of the recirculating water system. The agglomeration and accumulation of oversprayed paint in the water supply of the recirculating systems results in serious problems such as blockage of the pipes and pumps that circulate the water, and a build-up of paint on the walls of the booth. As more paint is sprayed in the booth, the oversprayed paint removed from the air builds up in the water in the form of a tar-like sludge that can plug the pumps and lines of the paint spray booth recirculating water system. Furthermore, this sludge is extremely difficult to remove from the pump, lines, reservoir, and other internal surfaces of the system. The accumulation of the oversprayed paint in the bottom of the water reservoir also creates a serious problem when the system is periodically cleaned out, with much effort being required to remove the heavy build-up of the oversprayed paint on the bottom of the reservoir.
In order to maintain efficient operation of paint spray booths, detackifying agents are commonly employed in the recirculating water system. Detackifying the paint eliminates or minimizes the tacky properties of the paint, thus preventing the oversprayed paint from adhering to the walls of the spray booth. For instance, to remove water-borne paint from the recirculating water, coagulating agents are added to destabilize the paint solution by reducing negative charge of the particle. The process of destabilizing such solutions is termed coagulation. Flocculation follows in this treatment process, where the destabilized particles are induced to come together, make contact and form large agglomerates.
Detackification also involves the adsorption of the material added to a paint particle, which is similar to a coagulant. However, not all detackifiers are recognized as coagulants. For example, inorganic clays are detackifiers but they do not perform as coagulants. This is because they do not have an overall cationic charge and therefore do not agglomerate particles, which generally have a negative charge. In addition, not all coagulants are effective detackifiers, since a detackifier after adsorption of the paint particle must also make the paint non-sticky.
Various inorganic compounds have been used as flocculating and coagulating agents in paint spray booth water treatment systems. For example, aluminum sulfate has been used widely as a coagulant in paint spray booth water. However, the use of inorganic agents has disadvantages such as high volumes of residual waste sludge. Moreover, inorganic agents are not effective for detackifying solvent paints. Therefore, it is very important to treat the water in the booth in such a way as to make the oversprayed paint material non-sticky so that it will readily separate itself from the water, not adhere to the walls, pipes, pumps and other internals of the spray booth system. It is also desirable to flocculate the paint particles to form a detackified floating sludge. And, it is also desirable to reduce residual harmful solvent.
Oversprayed paint contains various organic compounds, some of which are also hazardous, such as naphthalene and toluene, and these materials as well as the paint pigments, vehicle and resins require treatment. A convenient and officially accepted method to determine the organic content of wastewater and to estimate the efficacy of a process to reduce the amount of organic compounds is to measure the chemical oxygen demand using a method such as ISO 6060:1989 or similar. Chemical oxygen demand (COD) is based upon the fact that nearly all organic compound may be fully oxidized to carbon dioxide by use of a strong oxidizing agent such as chromic acid under highly acidic conditions. COD has long been used for such analyses and is a highly reliable method. COD is reported in milligrams per liter of oxygen required for oxidation of the organic materials in a given sample. The greater the demand, the greater the amount of organic material in the sample.
Other problems may also develop in spray booth systems. For example, the tacky paint deposits are excellent food sources for inadvertently introduced microorganisms such as bacteria and fungus. Often these inadvertently introduced microorganisms can cause odor problems and spot corrosion in the system.
The ultimate waste from the painting process in automobile manufacturing is in the form of sludge. As the sludge may contain hazardous chemicals such as naphthalene and toluene, sludge may be considered a toxic waste. Therefore, disposal of the sludge may present another problem to the industry. Not only does the disposal require more investment, but also the sludge to be disposed of must meet the stringent environmental regulations of government. Therefore, it is additionally desirable to remove solvent during detackification.
Since both the quality and quantity of the sludge determine the final disposal cost, it is desirable to remove hazardous chemicals and other biodegradable solids form the sludge as much as possible in order to reduce the disposal cost of the paint sludge. Thus a sludge devoid of hazardous chemicals can be considered a regular waste and, disposal cost would then be reduced, while a smaller volume of sludge produced would translate into reduced disposal cost.
In recent years, the need to reduce solvent emission has resulted in the reduction of solvent-based or solvent-borne paints, and an increase in the use of water-based or waterborne paints. Because the hydrophilic properties of the waterborne coating compositions render such compositions readily dispersible or soluble in water, removal of paint solids comprised of components such as organic resins, pigments, and organic solvents, from waterborne paint overspray typically requires the use of different detackifying processes in paint spray booths when compared to solvent-based paints. While solvent-based or solvent-borne paint use has been decreasing, it is still used in some markets and locales, although in most places environmental regulations require removal of the solvents in an environmentally acceptable way.
Although microorganisms have been used in spray booth paint detackification systems for some times, in some cases the COD reduction has been found to be inadequate or less than that desired. One reason for this problem is that in some spray booth operations, much more organic solvent is used and therefore much more organic solvent is trapped by the water in the spray booth overspray collection system. In some cases, the excess quantity of organics, including the additional solvent, has overwhelmed the capabilities of even the best microorganism-based paint detackification systems. This problem has been observed in paint operations which utilize several paint colors through a single nozzle or system of nozzles and in changing paint colors additional solvent is circulated through the nozzles to remove prior paint color. In such circumstances the microorganisms may be unable to cope with the excess organic load as fast as it is introduced into the water stream, resulting in the necessity to replace part or all of the recirculating water, creating an expensive and difficult disposal problem, since such contaminated water would be considered a hazardous waste.
These problems demonstrate the desirability of an effective novel paint detackification method that can minimize the deposition of sticky, oversprayed paint in the system, make the resulting paint sludge non-sticky and easy to remove, provide good-quality water that can be recirculated in the system, and that can withstand the higher levels of solvent in some paint overspray.